In recent years, exhaust gas standards for automobile engines have been getting more stringent from year to year, methods for engine evaluation have also been getting more demanding, and methods for performing evaluation during start and acceleration have been introduced. As specific evaluation methods, a JE05 mode has been implemented in Japan and an ETC mode has been implemented in Europe (EC).
Exhaust gas standards widely use nitrogen oxide (referred to hereinbelow as NOx) and diesel particulate (referred to hereinbelow as PM) values, but because of a trade-off relationship therebetween, it is extremely difficult to decrease both.
Presently, effective means for meeting the exhaust gas standards include the decrease in amount of NOx by a high exhaust gas recirculation (referred to hereinbelow as EGR) and the decrease in PM by ensuring a high lambda (extra intake coefficient λ). To meet both requirements, it is necessary to use a system that ensures a high lambda over the entire engine range (entire operation range), that is, a system that can supply high overcharge to the engine.
FIG. 3 illustrates a transient (JE05 mode) operation mode adapted in Japan and test data. Time (sec) is plotted against the abscissa, and smoke-lambda-engine revolution speed are plotted in the order of description from below on the ordinate.
As shown in FIG. 3, where a vehicle is started from an engine idle state, that is, from a state in which the vehicle has been stopped, the engine revolution speed rises rapidly when the vehicle is started. At this time, the amount of intake air is temporarily insufficient and lambda rapidly drops close to 1. As a result, a large amount of smoke is discharged.
A two-stage turbo system can be considered as a system capable of supplying high supercharge to an engine with the object of satisfying the above-described exhaust gas standards.
A two-stage turbo system is basically a system in which by disposing two turbochargers for an engine, it is possible to realize a supercharge that is higher than that in the conventional single turbo engine over the entire engine operation range.
In such two-stage turbo system, a high supercharge is realized by performing two-stage supercharging with two turbochargers (see Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-open No. 2004-225586.    Patent Document 2: Japanese Patent Application Laid-open No. 2005-315163.